There is frequently a need to image the internal, three-dimensional structure of objects, in order to analyze internal structures or mechanism of failure, particularly at the scale from 200 nm to 10 mm. Imaging methods based on the transmission of a probe beam through a material can be used to generate a tilt series of data for three dimensional reconstruction, but such methods are typically insensitive to structure parameters, and are limited in the resolution that can be provided. A potentially more informative and higher spatial resolution approach to the collection of tomographic data sets can be to physically section an object in many slices while collecting images at incremental sectioning steps through the object. This sectioning approach also enables the collection of maps of many structure parameters, such as composition, orientation, hardness, and resistivity. Further, surface treatments known to reveal microstructural information can be used at each step, expanding the range of structure parameters that can be mapped for each slice sectioning an object, and thus, for the entire object being examined.
A straightforward approach to sectioning an object for tomography is the mechanical removal of material of each slice to be imaged, known as serial sectioning. A major problem exists with the sectioning approach. For the mechanical sectioning approach, however, it is very difficult to accurately measure the depth of any particular step of material removal at small length scales. This is in part caused by the difficulty in maintaining a reference plane for an object being sectioned in a planar manner. Mechanical measurement systems that do not rely on a reference plane on the object being measured are limited to a resolution of one micron at best, rendering the sectioning only useful for objects with detail on the scale of at least twenty microns. That is, objects that are least two centimeters in size, or larger, can be mechanically sectioned with good resolution.
Other methods of mechanical measurement, such as surface profilometry, measure only relative changes in height, and therefore require a reference plane if absolute depth measurements are desired. The profilometry method is not practical when sectioning an object in a planar manner. Other mechanical systems allow automated removal of material from an object by cutting or grinding, controlling the advance of a machining head. These removal methods are designed to reach and reveal a certain location within an object. The accuracy of these removal methods is severely limited to five microns in resolution. Interferometry methods enable measurement of depths with a much finer resolution, but such interferometry systems require an external reference object to measure the amount of material that has been removed from an object. As such, the interferometry methods suffer from the need to have external references. Additionally, incorporation of an interferometry system into an automated sectioning or tomography system would add significant complexity and cost.
Commercial companies offer systems that automate some process steps in a sectioning process. Such systems typically use a robotic arm to iterate between sectioning, cleaning, and imaging, with the sectioning steps lasting for a predetermined period of time. Such automated systems and methods reduce labor but are limited in resolution accuracy while sectioning through material. For example, sectioning rates are approximated, and typically a human operator must tend the machine in order to stop the sectioning process when the desired region is observed in the transient displayed images generated during the process. In some cases, the feature of interest will have an appearance that is characteristic enough to allow for the robotic sectioning apparatus to stop automatically, but with most objects, such is not the case. It is desirable to have a more predictably automated system that can accurately determine depths of sectioning.
Sectioning machines are available that are capable of accurately sectioning material, but with a precision of only five microns, which is too coarse for many applications. Such sectioning hardware relies on mechanical sensors to measure depth. The sectioning machines can store data on material removal rates and sectioning headwear rates to compensate for tool wear and drift. This offers an improvement over the measurement of only the depth of material removed, but is limited because such compensation requires calibration runs and to generate data sets related to tool wear and material grinding rates. It is necessary to perform these calibration runs before sectioning any new object that differs substantially from any performed previously on an instrument. Also, replacement of consumables such as the sectioning head requires recalibration. The frequent need for recalibration severely limits the use of such sectioning machines for sectioning objects and necessitates a thorough and complicated set of calibration data retention and labeling.
Several US patents describe methods of detecting end points of depth sectioning procedures and methods of measuring the depth of material removal in grinding and polishing operations. U.S. Pat. No. 7,014,531 describes a detection mark that is intended to measure the depth of grinding at a given stage. Its precision is five microns, which is insufficient for tomography of objects less than 1000 microns in size. U.S. Pat. No. 6,734,427 describes a method that uses an object of predetermined size to mark a stopping point for a polishing operation. It does not measure the increment of polishing steps. U.S. Pat. No. 6,533,641 describes a similar resolution capability and involves mounting a sample on a steep incline and imaging during grinding and polishing. The ground and polished surface is not imaged, rather the incline is merely used to allow visual access to distinguishable features during grinding and polishing of the object. The incline in this patent is intended to allow visual access to all surfaces of a parallelipipedal object during sectioning, and does not measure depth. U.S. Pat. No. 6,121,147 discloses a reporting substance for detecting a polishing depth, but suffers from two major limitations. First, the reporting substance marks only an end point in the grinding and polishing procedure. Second, the method describes the use of spectroscopy in detection, which leads to the use of overly complicated spectroscopy devices than are necessary for this application. U.S. Pat. No. 5,077,941 describes a predetermined pattern of raised bumps of known geometry as part of a sensor that can serve as a coarse indicator of depth during a grinding and polishing procedure. As various bump features becomes visible, the sectioning depth can be coarsely and qualitatively estimated. This method requires the building of a complicated reporting device as well. Prior sectioning and tomography systems and methods suffer from complicated and limited collateral reporting and sensing devices, and used the sectioned material as a sectioning object for measurement during grinding and polishing. These and other disadvantages are solved or reduced using the invention disclosed herein.